1. Field of the Invention
The present invention relates to a method of fabricating a bit line. More particularly, the present invention relates to a method of fabricating a bit line used in ULSI.
2. Description of the Related Art
A memory is a semiconductor device used to store information or data. In the storage of the digital information, the memory capability of a memory device is measured in units of a "bit". Each unit for storing information in the memory is called a memory cell. The memory cells are typically arranged in a matrix corresponding to a set of two coordinates such as (column, row). Each coordinate set represents the unique address of a memory cell. The memory cells located in the same column or the same row are connected by the same conducting wires. While transverse conducting wires connected to the memory cells located in the rows are called word lines, the longitudinal conducting wires for data transmission are called bit lines.
The conventional bit lines are made of doped polysilicon and tungsten silicide. The method of fabricating conventional bit lines is to form a dielectric layer on a substrate to isolate the bit lines and the substrate. A metal is replaced by polysilicon because the adhesion between the metal and the silicon is poor. However, even the polysilicon is doped to increase conductivity thereof, the resistivity of the polysilicon is still high. A silicide layer is thus formed on the polysilicon layer to take advantage of the better conductivity of the silicide. The conducting wires are the conductive layers formed by the polysilicon layer and the silicide layer. The bit lines are formed by a photolithography process, after which the bit lines are covered a dielectric layer. The bit lines are isolated from other devices.
As the integration of semiconductors is increased, the size of the devices is reduced. For example, the feature size of the bit line should be below 0.15 .mu.m for fabricating 0.21 .mu.m DRAM. Because of the limitations imposed by photoresist resolution, it is difficult to achieve the object by conventional photolithography processes unless a more advanced stepper machine is used. However, manufacturing costs will be increased by changing to the advanced stepper machine.